Surfaces of various products can look dirty, cause sanitary problems, or demonstrate performance degradation caused by corrosion or the like, due to exposure to various contaminating substances from the environment. Among such products, heat exchangers in air conditioning equipment are highly susceptible to contamination from the environment due to the functions thereof, and the contamination easily causes a variety of failures.
A heat exchanger has a structure in which a large number of fins (for example, aluminum fins) are attached to a pipe in which a coolant flows, and heat exchange efficiency is increased by fins with a large surface area. Condensed water can easily adhere to the fin surface during cooling and warming, and the ventilation resistance can increase and heat exchange efficiency can decrease due to a phenomenon of the fins being connected to each other by the condensed water (referred to hereinbelow as “bridge phenomenon”). In particular, since the bridge phenomenon is easily induced by contamination such as dust adhering to the fin surface, the bridge phenomenon is typically prevented by forming an organic or inorganic hydrophilic coating that excels in antifouling performance on the fin surface. In the present description, the “antifouling performance” means the performance such that contamination is unlikely to adhere and the adhered contamination is easily removed.
However, when an inorganic hydrophilic coating contains a large amount of an inorganic component such as water glass or boehmite, the coating easily adsorbs odor. For this reason, organic hydrophilic coatings are often used. Meanwhile, organic components of organic hydrophilic coatings are easily decomposed, degraded, or dissolved in condensed water with the passage of time, and the antifouling performance or hydrophilicity of the hydrophilic coating are degraded.
Accordingly, techniques for forming hydrophilic coatings by using various coating materials have been suggested for preventing the degradation of antifouling performance and hydrophilicity of hydrophilic coatings. For example, Patent Document 1 suggests a method for forming a hydrophilic coating by using a coating material comprising a modified polyvinyl alcohol and a crosslinking agent. Patent Document 2 suggests a method for forming a hydrophilic coating by using a coating material comprising carboxymethyl cellulose, polyethylene glycol, and a crosslinking agent. Patent Document 3 suggests a technique for forming a thin film constituted by ultrafine silica particles and fluororesin particles. By using a configuration in which hydrophobic portions are present in a spot-like manner on a hydrophilic surface, it is possible to prevent the adhesion of hydrophilic or hydrophobic contaminants of various properties, while maintaining high hydrophilicity of the entire film. Since the film is thin, the problem of odor adsorption can be avoided and the film can be adapted to a heat exchanger.